A. MATHEMATICS AND ASTRONOMY. 17 



and a repulsion was actually exhibited. Unfortunately, how- 

 ever, there was some difference of opinion about the inter- 

 pretation of the phenomena, and the decisive test has yet to 

 be applied. 



SPECTRUM OF EXCKE'S COMET. 



Professor Harkness, in a communication on Encke's comet, 

 states that ever since November 18 it became steadily 

 brighter and brighter, and its spectrum more distinct. On 

 the evening of December 1 the spectrum consisted of three 

 bright bands ; the most refrangible one being very faint, the 

 middle one by far the brightest, and the least refrangible one 

 having a degree of brilliancy intermediate between that of 

 the other two. The shape of each of these bands somewhat 

 resembled an isosceles triangle, with its base turned toward 

 the red end of the spectrum. In the case of the two bright- 

 er bands the light increased quite rapidly from the less re- 

 frangible edge of the band, until it attained its maximum at 

 a point distant from that edge by about one quarter of the 

 whole breadth of the band, and thence it gradually faded 

 away toward the more refrangible edge of the band. In the 

 case of the most refrangible band, the light seemed to be of 

 nearly equal intensity throughout its whole breadth. The 

 positions of the two brighter bands were measured, and the 

 resulting wave-lengths of the light, expressed in millionths 

 of a millimetre, are approximately as follows : First band, 

 less refrangible edge, 556 ; brightest part, 550.0 ; more re- 

 frangible edge, 534: second band, less refrangible edge, 515; 

 brightest part, 510.9 ; more refrangible edge, 499. The posi- 

 tion of the faint band was estimated, and the resulting wave- 

 lengths are, for the less refrangible edge, 458, and for the 

 more refrangible edge, 448. At times he fancied he also saw 

 a faint continuous spectrum, but could not satisfy himself 

 that it really existed. 



Both in appearance and wave-lengths this spectrum bears 

 such a remarkable resemblance to that of the second comet 

 of 1868, that Professor Harkness is strongly inclined to think 

 their physical constitution must be identical. It will be re- 

 membered that the observations of Dr. Huggins showed that 

 the spectrum of the latter comet was the same as that of ole- 

 fiant gas. 



